1. Field of the Invention
The present invention relates to an ultraviolet-erasable nonvolatile memory apparatus (EPROM: Erasable Programmable Read Only Memory), and more particularly to a nonvolatile memory having a special information memory cell for keeping special information, which is undesirable to be erased at the time of illumination of an ultraviolet ray, in addition to having an ultraviolet-erasable memory cell mainframe.
2. Description of the Prior Art
FIG. 4 is a cross-sectional view showing an arrangement of a special information memory cell of such a conventional nonvolatile memory apparatus. In the illustration, numeral 2 represents an aluminium (Al) coat provided in order to prevent the transmission of an ultraviolet ray and 3 designates a passivation film for protecting a wiring pattern, the Al coat 2 and others from being insulated or rusted. Further, numerals 4 to 9 are components of the memory cell, 4 denoting an oxide film, 5 depicting a control gate, 6 designating a floating gate, 7 indicating an n.sup.+ source (or n.sup.+ drain), 8 designating an n.sup.+ drain (or n.sup.+ source) and 9 being a P-type silicon substrate. Here, the wiring pattern is formed on the oxide film 4.
In operation, in FIG. 4, when ultraviolet illumination is effected in order to erase the information of the memory cell main frame, not shown, of the EPROM, the special information memory cell shown, i.e., the memory cell of the EPROM portion (UPROM: Unerasable PROM) undesirable to erase the information, is covered by the Al coat 2 to prevent the transmission of the ultraviolet ray, thereby preventing the erasure of the information of the memory cell comprising the oxide film 4, control gate 5, floating gate 6, n.sup.+ source (or n.sup.+ drain) 7, n.sup.+ drain (or n.sup.+ source) 8 and P-type silicon substrate 9. Here, the special information unerasable irrespective of the ultraviolet illumination is used for the wafer test and others in a semiconductor memory manufacturing process, for example, is laser trimming information, test information, wafer history information or the like.
Since such a conventional nonvolatile memory apparatus is constructed as described above, for preventing the short-circuit to the wiring pattern, there is a problem that it is necessary to sufficiently provide a space around the Al coat 2 so as to restrict the formation of the wiring pattern. Further, if reducing the area of the Al coat 2 to soften the restriction on the formation of the wiring pattern, the ultraviolet ray radiated can pass around the Al coat 2 to reach the memory cell, thereby making the operation unstable and lowering the reliability.